Add socket activation for RHEL based distributions

Removes the systemd drop-in unit file for socket activation and instead
prefers socket activation by default for both RHEL based and DEBIAN
based distributions.

Socket activation for RHEL based distributions was tested on CentOS 7 and Fedora 28.

Signed-off-by: Eli Uriegas <eli.uriegas@docker.com>

contrib/init/systemd/docker.service

@@ -10,7 +10,7 @@ Type=notify
 # the default is not to use systemd for cgroups because the delegate issues still
 # exists and systemd currently does not support the cgroup feature set required
 # for containers run by docker
-ExecStart=/usr/bin/dockerd
+ExecStart=/usr/bin/dockerd -H fd://
 ExecReload=/bin/kill -s HUP $MAINPID
 TimeoutSec=0
 RestartSec=2

systemd/00-socket-activation.conf

@@ -1,7 +0,0 @@
-[Unit]
-After=docker.socket
-Requires=docker.socket
-
-[Service]
-ExecStart=
-ExecStart=/usr/bin/dockerd -H fd://